1. Field of the Invention (Technical Field):
The present invention relates to the use of solar energy for the processing of gases, particularly for the thermal dissociation of gases, and specifically for a method and apparatus for processing slightly-absorbing or transparent gases, including CO2, CH4, H2O and combinations thereof, and others.
2. Background Art
The industrialized world presently relies heavily on fossil fuels for its energy. The very large-scale combustion of these fuels causes a number of environmental concerns, including accumulation of carbon dioxide (CO2) in the Earth's atmosphere. Much of the demand is for convenient-to-use liquid and gaseous fuels. The present invention is of solar reactor designs to enable the deposition of solar energy in gases, leading to their endothermic dissociation into smaller molecules that are the starting point of synthesis of convenient liquid and gaseous fuels. An example of such a process is the direct solar reduction of CO2 to carbon monoxide (CO) and molecular oxygen (O2), as disclosed in U.S. Pat. No. 6,066,187 and in Traynor, A., et al., “Direct Solar Reduction of CO2 to Fuel. First Prototype Results,” Ind. Eng. Chem. Res. 2002, 41, 1935–1939. In general, the process includes molecular dissociation, which can be accomplished by heating or by the absorption of a sufficiently energetic photon, followed by the extinguishing of the back reaction. At the critical point in the process, high energy products exist in reactive mixtures at high temperatures. It is necessary to arrest such reactions by suddenly dropping the temperature, diluting the mix, or both. This rapid extinguishing of the back reaction is referred to as “quenching.”
Some gases absorb or have inducible absorption in the solar region of the spectrum. The gases of interest to the present invention are slightly absorbing at most, and will be referred to as such. Carbon dioxide is an example of this class of gases. Normally, it has no absorption bands in the solar ultraviolet or visible spectral regions. But if it is radically heated it does begin to absorb in these regions, as explained further in Jensen, R., et al., “The Ultraviolet Absorption Spectrum of Hot Carbon Dioxide,” Chem Phys Lett. 1997, 277, 356, and in U.S. Pat. No. 6,066,187. Other molecules of interest share this property. Almost all of these slightly absorbing molecules develop increased absorption in the visible spectrum if preheated. For these gases, strong preheating is necessary before absorption of sunlight is possible. The present invention provides an apparatus and method that preheats and then exposes the subject gas to a maximum intensity of direct solar photons to perform heating and photolysis.
Molecules that are non-absorbing in the visible spectrum and show little or no enhancement of absorption upon preheating are termed “transparent.” Such gases require a specialized type of reactor. In their case, energy must be added to the molecule either by contact with a hot surface or by absorption in the infrared region of the spectrum. All of the molecules of interest have strong absorption in the infrared. For each case it is possible to heat them through infrared photon absorption. This preferably is accomplished by an apparatus we refer to as a “holraum reactor.” The holraum reactor uses a process of absorbing visible light into a solid, which can then re-radiate the energy in a spectrum that is shifted toward the infrared.
U.S. Pat. No. 6,066,187 is useful as a description of the prior art. The '187 patent contemplates a very large funnel accepting highly focused light from thousands of square meters of mirrors with a huge focal spot. For reasons both technical and economic, there is a need for an apparatus of a much smaller size, for example 100 square meters of reflective surface with a five or six inch spot.